Tall, dark and handsome: the formula for romantic heroes is simple.
Men need to be attractive, powerful and unattached. They can be anything from billionaires to gardeners to doctors - but they are always mysterious.
At a push they might have a wife they no longer love (and is hidden in the attic) but under no circumstances will they cancel a hot date to look after their snotty toddler.
Or at least, that used to be the case. This summer, according to romance novel veterans Mills&Boon, the most desirable male heroes in fiction are hot, single dads.
Women in their droves are apparently buying beach reads that centre around these unattached but oh-so domesticated chaps.
"There are so many different kinds of romance heroes, but single dads have a nurturing and caring element that is very appealing," a spokesperson for Mills&Boon told Mail Online.
"The current popularity of the single dad trend definitely has some caring family men, but readers are appreciating these dad heroes in a new sexier light - from "cute father" to "hot daddy" if you will."
Brand marketing director Joanne Rose added: "Our 'single dad' stories certainly strike a chord with readers - the heroes are real and relatable, and often offer hope of a second chance at happily ever after. "
Some of the latest books from Harper Collins' romance imprint include Emily Forbes' Falling for the Single Dad, in which the hero is a plastic surgeon returning from active duty in Afghanistan; to Charlene Sands' The Billionaire's Daddy Test, which sees a woman tries to find the (single) father of her dead sister's child.
Other new titles include The CEO Daddy Next Door (woman finds herself with a billionaire single dad neighbour)', Bachelor Dad (single mum falls for single dad), The Daddy Project (single mum and dad have fake relationship) and Saved by the Single Dad (paramedic dad is saved by red-headed beauty).
All have covers featuring images of topless men with six-packs.
But the concept of the 'hot, single dad' is not a new one. On social media, the hashtag #DILF (Dad I'd Like To...) has been going for years, while there are numerous accounts dedicated to #DilfofDisneyland,#DilfOfTheDay and #DilfsOnTour.
The appeal of the DILF is simple: they still tick the tall, dark and handsome box, but the fact they have children also proves they are capable of commitment and emotional maturity. What more could any woman want?
In fiction, the hot, single dad is flawless. He might be widowed and struggling to cope with his grief, or his cruel ex walked out on him.
In real life, one might wonder whether he played in part in his singledom: Did he cheat?
Was he selfish, unhelpful and inconsiderate? Did he call it 'babysitting' when he took care of his kids? There are plenty of glaring pitfalls when dating a single dad, but in romance novels, none of these matter.
So long as he's sexy and willing to change his one superficial flaw, then it's a recipe for love.
The hot, single mum (spinning endless plates but still looking flawless) is yet to receive a category of romance as blatant as the single dad - unless you're on a porn website.
What an oversight. Forget single dads - who wouldn't want to read about Bachelor Mum, The CEO Mummy Next Door or Saved by the Single Mum?. Roll on summer 2018, we say.